Unknown Coordinate System of 1900s USGS Topographic maps

I have 1890s & 1900s vintage scanned Topographic Maps. The Coordinate system is obviously a Geographic Coordinate system since only the lat/longs are represented in the tics. the datum is Mean Sea Level. Perhaps they are in Polyconic Projection and Clarke 1888 ellipsoid. based on the corner Tic coordinates they were geo-referenced to Datum NAD27 because of convenience.

I am not an expert in geodesy, but how much shifts are expected for such transformations? Is such practice acceptable? There seems to be no sure answer. Any guidance/help is greatly appreciated.

Okay, the map's 1:125000 and there's very little detail that would work for georeferencing. I retract my earlier statement! You may have to use the corner points as the best you've got and call it NAD27. If you can identify a few other interior points, that might help--but the reference data should also be in NAD27, not NAD83 or WGS84.

They predate NAD27, so they're not on that GCS/datum. There was a New England datum, but there are no transformations from it to anything else that I'm aware of. The actual coordinate system of the map is unlikely to be geographic aka latitude/longitude, but they annotated it with a graticule and not with the grid (projected coordinate system) information.

You should probably try to georeference it with physical points like buildings or possibly street intersections, not the lat/lon tics.

What exactly are you trying to accomplish? I assumed you need to georeference an existing scanned topo and presented an alternative to download a pre-georeferenced one. Melita also gave you an answer, but without being clearer about what you need, I don't think we can help further.

Okay, the map's 1:125000 and there's very little detail that would work for georeferencing. I retract my earlier statement! You may have to use the corner points as the best you've got and call it NAD27. If you can identify a few other interior points, that might help--but the reference data should also be in NAD27, not NAD83 or WGS84.